On the Mechanisms of pH-dependent Hydrogen Exchange of Bovine Plasma Albumin in the Range of pH 5 to 8.5

Abstract

Hydrogen exchange rates of bovine plasma albumin vary over the range of pH 5 to 8.5. As the pH is raised, the number of very slowly exchanging hydrogens decreases and the number of rapidly exchanging hydrogens steadily increases. The rate of exchange of hydrogens of the very slowly exchanging core at pH 5 was studied at pH values from pH 5.2 to pH 8.5. The results obtained suggest that this protein is highly motile; that is, it fluctuates rapidly at pH 7 and above between states in which exchangeable hydrogens are accessible to bulk solvent and ones in which they are not accessible. In the range of pH 5 to 6.5, no segments of the core appear to be highly motile but compact segments may become highly motile as the pH is raised through this range. The alternative possibility, that changes in exchange behavior at pH 5.2 to 6.5 are the results of changes of the local environment of individual exchanging groups, cannot be excluded. The effects of sodium dodecyl sulfate, glycerol, and changing ionic strength were also studied at pH 5 and 7.7. The results were consistent with three possible explanations: (a) that these agents stabilize compact conformational states of segments of bovine plasma albumin; (b) that they reduce segmental conformational motility;; or (c) that they alter the local environment of individual exchanging groups and thus influence the rate dependencies of exchange of these units. Discrimination between these three mechanisms was not deemed possible on the basis of our experimental data.